Not applicable.
1. Field of the Invention
This invention relates to shelled Pd-Au catalyst of particular characteristics, and methods for their production, which are effective for catalyzing the vapor phase reaction of an alkene (such as ethylene) with an alkanoic acid (such as acetic acid) and oxygen to produce an alkenyl alkanoate (such as vinyl acetate) at high values for space-time yield, specific activity, and with a high selectivity for conversion of the alkene to the alkenyl alkanoate (such as ethylene to vinyl acetate).
2. Description of the Related Art
Vinyl acetate (VA) is a commodity chemical in high demand as a monomer for production of poly(vinyl acetate). This important polymer, and its derivatives, finds extensive uses as adhesives, paints and other coatings, films and laminating materials. Many techniques have been reported in the prior art for the production of VA. A chief technique is a catalyzed gas phase reaction of ethylene with acetic acid and oxygen. Today a type of catalyst widely use for this reaction is a surface shell impregnated catalyst of a type as described in U.S. Pat. No. 4,048,096 by T.C. Bissot.
Bissot""s U.S. Pat. No. 4,048,096 discloses a catalyst having a specific activity of at least about 83 grams of vinyl acetate per gram of precious metal (Pd+Au) per hour measured at 150xc2x0 C. and a reaction pressure of 120 psig. The catalyst consists of: (1) catalyst support particles having a particle diameter of from about 3 to about 7 mm and a pore volume of from about 0.2 to about 1.5 ml/g, (2) palladium and gold distributed in a surface layer of the catalyst support extending less than about 0.5 mm into the support, the palladium being present in an amount of from about 1.5 to about 5.0 grams per liter of catalyst, and the gold being present in an amount of from about 0.5 to about 2.25 grams per liter of catalyst, and (3) from about 5 to about 60 grams per liter of catalyst of an alkali metal acetate. Palladium is the active catalyst metal and the gold is a catalyst promoter.
The Bissot ""096 patent process for catalyst preparation comprises: (1) impregnating the catalyst support with an aqueous solution of water-soluble palladium and gold compounds, (2) precipitating water-insoluble palladium and gold compounds on the catalyst support surface by contacting the impregnated catalyst support with a solution of compounds (preferably sodium metasilicate) capable of reacting with the water-soluble palladium and gold compounds to form water-insoluble palladium and gold compounds, (3) converting the water-insoluble palladium and gold compounds into palladium and gold metal on the support surface by treatment with a reducing agent, (4) washing the catalyst with water, (5) drying the catalyst, (6) impregnating the catalyst with an alkali metal acetate promoter (e.g., a potassium promoter), and (7) drying the catalyst.
The improvement disclosed in Bissot ""096, as compared to prior Pd-Au supported catalysts, involves distributing the catalyst loading of palladium and gold as a surface layer on the catalyst support which is less than about 0.5 millimeter into the support from its surface. The impregnating step is carried out with an aqueous solution of palladium and gold compounds and the total volume of the solution is from about 95 to about 100% of the absorptive capacity of the catalyst support. The precipitating step in Bissot is carried out by soaking the wet catalyst support with a solution of an alkali metal silicate, the amount of alkali silicate being such that, after the alkali metal silicate solution has been in contact with the catalyst support for about 12 to 24 hours, the pH of said solution is from about 6.5 to about 9.5. In all examples of Bissot the reduction of the precipitated compounds to Pd and Au metals is accomplished by reaction with a hydrazine solution.
As is apparent from a reading of the Bissot patent, a major concern in this art of vinyl acetate (VA) production has always been to improve the space-time yield (STY) and also the specific activity (SA) of the catalysts. Since the description of this shell type of catalyst by Bissot others have attempted to improve the catalyst in respect to its space-time yield, specific activity, and/or its selectivity
In U.S. Pat. Nos. 5,179,056; 5,189,004; and 5,342,987 by W.J. Barley it is reported that a shell impregnated catalysts of the Bissot type is improved in respect to its STY if it is essentially free of sodium; such as if it prepared from ingredients that are essentially free of sodium as per the ""056 patent, or if its sodium content is removed by washing with water or an aqueous solution of a potassium promoter as in the ""004 patent, or by washing the catalyst at an intermediate stage of its production with an ion exchange solution as in the ""987 patent. In all of the above patents the exemplified catalyst are reduced with hydrazine solutions. U.S. Pat. No. 5,693,586 reports that a shell impregnated catalysts of the Bissot type which are made from reagents that are all potassium salt compounds are of an improved carbon dioxide selectivity. In this patent all example catalyst are reduced with ethylene at a temperature of 150xc2x0 C.
Barley et al. in U.S. Pat. No. 5,274,181 reports that a shell impregnated catalysts of the Bissot type is improved in respect to its STY if it is prepared to have, at a Pd loading of 2.5 g/L (0.33 wt %) to 6.1 g/L (1.05 wt %), a weight ratio of Au to Pd in the range of 0.6 to 1.25. All catalyst examples of this patent are reduced by reaction with a hydrazine solution.
U.S. Pat. No. 5,567,839 reports that a shell impregnated catalysts of the Bissot type is improved in respect to its STY if a barium salt rather than a sodium silicate is use to precipitate the Pd and Au compounds into the shell. All catalyst examples of this patent are reduced by reaction with a hydrazine solution.
The selectivity of a palladium-gold catalyst in vinyl acetate synthesis also is influenced by the extent and uniformity of the palladium metal and gold metal distribution on the exterior and/or interior surfaces of a porous catalyst support substrate, such as carbon dioxide selectivity and oxygen conversion in an ethylene, acetic acid and oxygen vapor phase reaction.
Attempts to provide a uniform distribution of the palladium and gold metals on the catalyst support has involved manipulation of the catalyst preparation steps and/or by using support substrates having various specified pore dimensions. Particularly useful improvements in preparing highly active catalysts for vinyl acetate production are disclosed in U.S. Pat. No. 5,314,858 and U.S. Pat. No. 5,332,710. These references describe process embodiments for improving palladium and gold distribution on a support by manipulating the precipitation step in which the water-soluble precious metal compounds are fixed to the support surface as water-insoluble compounds. In U.S. Pat. No. 5,314,858, fixing precious metals on the support is achieved utilizing two separate precipitation stages to avoid using large excesses of fixing agent. U.S. Pat. No. 5,332,710 describes fixing the precious metals by physically rotating an impregnated catalyst support while the impregnated support is immersed in a reaction solution at least during the initial precipitation period. The rotation immersion procedure yields catalysts in which the metals precipitated on the carrier are said to be more evenly distributed in a thin shell on the support surface. All catalyst examples of these patents are reduced with ethylene at a temperature of 150xc2x0 C.
Despite such improvements as have been made there is a continuing interest in the development of catalyst compositions that exhibit an even further improved combination of properties for the production of vinyl acetate.
This invention relates to a shell impregnated catalyst of Pd-Au, and methods for their production, which are effective for catalyzing the vapor phase reaction of ethylene with acetic acid and oxygen to produce vinyl acetate at high values for space-time yield, specific activity, and with a high selectivity for conversion of ethylene to vinyl acetate.
The shell impregnated catalyst of Pd-Au are produced on a silica support to have a Pd loading of 1.8 to about 7.2 g/L of catalyst and a Au to Pd weight ratio of 0.3 to 2.0 by impregnating the support with aqueous solutions of palladium and gold salts or acids, which preferably are high purity potassium tetrachlorpalladate (99.99%) and hydrogentetrachlorauate (99.998%), and thereafter precipitating water insoluble compounds of Pd and Au on the support surface by reaction of the impregnated support with solutions using alkali metal metasilicates or alkali metal hydroxides or mixtures thereof as precipitating agents, preferably a sodium metasilicate solution is used as a precipitating agent in a quantity that exceeds the theoretical amount required to neutralize the Pd and Au salts. The excess of fixing agent depends on volume of fixing solution and acidity of support.
The impregnated support is then washed with deionized water until the final decant is negative to a silver nitrate test, after which it is dried for water removal. The dried support with its surface precipitated compounds of Pd and Au is then reacted with ethylene or hydrogen at a temperature greater than 150xc2x0 C. and for ethylene up to 310xc2x0 C. and for hydrogen up to 299xc2x0 C., preferably for 10 minutes to one hour at a temperature of from 250xc2x0 C. to 325xc2x0 C. for ethylene and from 250xc2x0 C. to 299xc2x0 C. for hydrogen, until substantially all of its content of Pd and Au are reduced to their free metal state, after which the support is impregnated with potassium acetate to an extent of 6 to 7 weight percent of the total catalyst weight on a dry basis. Thereafter the catalyst is dried.
A catalyst as described above has a space-time yield (STY) and specific activity (SA) about 20-30% greater than an otherwise identical catalyst composition that is reduced with ethylene or hydrogen at 150xc2x0 C. In a temperature range of 140xc2x0 C. to 160xc2x0 C. at a gas hourly space velocity of 4500/hr the catalyst will exhibit a vinyl acetate selectivity of 90% or greater when operated under reaction conditions that result in a STY of at least 600 gVA/L catalyst/hr. Further, such catalysts have a long operational life.
This invention comprises a catalyst for the promotion of a gas phase reaction of an alkene, an alkanoic acid, and an oxygen-containing gas to produce an alkenyl alkanoate. The catalyst is particularly desirable for the gas phase reaction of an ethylene, an acetic acid, and an oxygen-containing gas to produce vinyl acetate.
In the catalyzed gas phase reaction process, ethylene reacts exothermically with acetic acid and oxygen in the vapor phase over a heterogeneous Pd-Au shelled catalysts, giving vinyl acetate and water:
CH2=CH2+CH3 CO2 H+xc2xdO22xe2x86x92CH3 CO2CH=CH2+H2 O, xcex94H=xe2x88x92178 kJ/mol. 
The vinyl acetate reaction process may typically operate at 140-180xc2x0 C., 5-10 atmospheres (atm), and a gas hourly space velocity (GHSV) of xcx9c 4500 hxe2x88x921. This will give 8-10% ethylene and 15-40% acetic acid conversion. Oxygen conversion can be up to 90%, and the yields are up to 99% and 94% based on acetic acid and ethylene, respectively.
Reaction temperatures may be between 140xc2x0 C. and 200xc2x0 C. Generally the preferred reaction temperature range is 140xc2x0 C. to 180xc2x0 C. with 140-160xc2x0 C. being most preferred. At temperatures below 140xc2x0 C., the reaction rate is low and it is difficult to keep the acetic acid in the vapor phase. Above 180xc2x0 C., for a given catalyst, more and more of the ethylene and acetic acid feeds are converted to by products. The principal by product is carbon dioxide. Generally, the other by-products, acetaldehyde, and ethyl acetate are formed at about 1% or less.
Reaction pressures are between 70-200 psig. Typically, the pressure used in commercial plants is 100-150 psig. Higher pressures make it difficult to keep the acetic acid in the vapor phase whereas pressures lower than 70 psig too greatly reduce the STY of the reaction.
The total volume of reaction gases as a gas hourly space velocity (GHSV) is about 3000-5000 STP liter/liter of catalyst per hour. Higher GHSV values result in higher STY and SA values without significantly lowering the selective values for production of vinyl acetate. Therefore, higher GHSV values, such 4500, are preferred. The composition of the reaction gases in volume % is in the range of ethylene, 27-60%; inerts 15-55%; acetic acid 12-17% and oxygen 6-8%. The reaction is operated with a large excess of ethylene and acetic acid. The main reason for doing so is to avoid formation of potentially flammable/explosive mixtures. Oxygen levels above about 9% are not used in order to avoid explosive mixtures. The preferred ranges, respectively are ethylene 50-60%, inerts 20-50%, acetic acid 12-15%, and oxygen 6-7%. Commercially, oxygen is often used in place of air and the percentage of ethylene in the feed is raised.
The support particles used in the process of producing catalyst of this invention are solid particulate materials that are capable of being impregnated with palladium, gold and a potassium promoter and that are inert under the conditions used to produce alkenyl alkanoates, such as vinyl acetate. Illustrative of such support particles are particulate silica, alumina, and silica-aluminas. Silica is the preferred support. The support preferably has a surface area from 100 to 800 square meters per gram. Silica beads of an average diameter of 5 to 6 mm, a surface area of 150 to 200 square meters per gram and a pore volume of 0.6 to 0.7 ml/g, such as xe2x80x9cKA-160xe2x80x9d sold by Sud Chemie AG, is an example of a most preferred support material.
The aqueous solutions of water-soluble palladium and gold compounds used in the process of this invention may include aqueous solutions of any suitable palladium or gold compound such as palladium (II) chloride, alkali earth metal tetrachloropalladium (II), palladium (II) nitrate, palladium (II) sulfate, gold (III) chloride or auric (III) acid (HAuCl4). However, compounds containing sodium are less preferred and the preferred compounds are potassium tetrachlorpalladate and hydrogentetrachlorauate. Then, for obtaining a high value for the space-time yield (STY) and specific activity (SA) of the catalyst it is preferred to utilize these preferred compounds in their high purity form, meaning 99.9+%purity, preferably 99.99%. Hence, it is preferred to use a potassium tetrachloropalladium of 99.99% purity and hydrogentetrachlorauateof 99.998% purity.
The quantity of Pd and Au compounds to be employed is such as to provide in the final catalyst a Pd loading of from about 1.8 g/L to about 7.2 g/L and a Au loading that places Au in the catalyst in a weight ratio to Pd in the range 0.3 to 2.0. Preferably the quantity of Pd loaded in the catalyst is such to provide the catalyst with a specific activity of greater than 200 g VA/g Pd/hr when operated under reaction conditions of 120 psig and within a temperature range of about 140xc2x0 C. to about 160xc2x0 C. that provide a STY of at least about 600 gVA/L cat/hr. The lower is the Pd loading that can be used to obtain the requisite STY values the higher will be the selectivity of conversion to VA, hence Pd loadings in a range of about 3.0 g/L to about 5.4 g/L are preferred.
The catalyst carries from 0.5 to 1.2 wt. % Pd. The catalyst carries from 0.3 to 2.4 wt. % Au. The catalyst carries from 1.0 to 4.5 wt. % potassium.
The support is impregnated in a process designated as xe2x80x9crotation immersion to a point of incipient wetness.xe2x80x9d The volume of the impregnation solution preferably corresponds to from 95 to 100% (more preferably from 98 to 99%) of the pore volume of the support. In this process, the catalysts support is immersed in the Pd-Au impregnation solution and tumbled or rotated therein during the initial stages of the impregnation of the support with the soluble precious metal compounds. The rotation or tumbling of the supports in the solution should proceed for at least 15 minutes and, preferably, for at least 30 minutes until all of the solution is absorbed. The rotation can last as long as up to 2 hours after which the support may be left without rotation inside the sealed container for one or two hours to complete distribution of the impregnation solution inside the support pores.
Any type of rotation or tumbling equipment can be used as the exact apparatus utilized is not critical. However the extent of the rotating motion may be critical. The rotation should be fast enough so that all surfaces of the impregnated supports are evenly contacted with the impregnation solution as soon as possible. The rotation should not be so harsh that actual abrasion of the support particles takes place. Generally, the extent of rotation should be about 1 to 30 r.p.m. and possibly even higher especially in the beginning of rotation depending upon the exact support utilized, the amount of support and the amount of precious metal to be impregnated into the support. The rpm to be used is variable and may also depend upon the apparatus utilized, the size and shape of the support, the type of support, and metal loading.
The precipitating agents used in the process of the present invention catalysts include sodium, lithium and potassium silicates and hydroxides. It is preferred to use sodium metasilicate as the precipitating agent. The precipitating agents are preferably employed in the form of aqueous solutions containing a 1.1 to 2.5 molar excess of the precipitating agents depending on support acidity and volume of used solution. The volume of such solutions used is preferably just sufficient to cover the support particles. The impregnated support is immersed into the fixing solution and allowed to remain completely covered (for 1 day up to about 3 days (xcx9c70 hours)) at room temperature until a final pH value of 6.5-8.8 is attained. The exact quantity of alkali, time of fixing and final pH is dependent on the alkali type, the acidity of the support, and the quantities of precious metals used.
After fixation is completed the impregnated support beads are then removed from fixing solution and rinsed with deionized (D.I.) water. Further washing may then done in a batch or a continuous mode. Further washing at room temperature should continue until the decant wash water content of chlorine ions is below 100 ppm, wherein the final decant gives a negative result to a silver nitrate test.
After washing is complete the impregnated support beads are dried, such as at 90-150xc2x0 C. in a forced air or nitrogen oven.
The reducing agent used in the process of this invention is ethylene or hydrogen to which the dried impregnated support are exposed while at a temperature greater than 150xc2x0 C. and up to 310xc2x0 C. for ethylene and up to 299xc2x0 C. for hydrogen, preferably of or greater than 200xc2x0 C. and more preferably to a temperature greater than 250xc2x0 C. such as a range of 275xc2x0 to 310xc2x0 C. for ethylene and 275xc2x0 C. to 299xc2x0 C. for hydrogen, and most preferably at 300xc2x0 C. for ethylene and 299xc2x0 C. for hydrogen, for a time sufficient to complete the reduction of Pd and Au to their free metal state. Generally, the reduction is carried out for no longer than five hours, and preferably less than one hour, preferably about 10 to 60 minutes.
Reduction of the impregnated support at these high temperatures has been found to significantly increase the activity of the catalyst as compared to catalyst synthesized from impregnated support that are reduced at lower temperatures such as 150xc2x0 C. as has typically heretofore been use in an ethylene or hydrogen reduction step. The higher reduction temperatures increase the STY of the catalyst by about 20-30% compared to one formed from impregnated support reduced at 150xc2x0 C. Thus, for catalyst compositions that are otherwise identical and of a calculated 0.6 wt. % Pd loading and a calculated 0.5% wt. Au loading and operated to provide a STY of 600 g VA/L cat/hr, the catalyst synthesized from impregnated support reduced at 300xc2x0 C. can be operated at a 8-10xc2x0 C. lower reaction temperature wherein the selectivity for conversion to vinyl acetate is 92-93% rather than the 90% range of one reduced at 150xc2x0 C.
For purposes of their reduction the impregnated support beads may first be heated in a flow of an inert gas, such as a nitrogen flow, from room temperature to 150xc2x0 C. The impregnated support may then be held in the inert gas flow at 150xc2x0 C. for 0.5-1 hour. Adsorbed water evolves during this heating period. The temperature may then be raised to 299-300xc2x0 C. Then the inert gas flow may be maintained and a volume of ethylene or hydrogen (1-5% by volume, preferably) may be introduced into the inert gas flow to form the reducing gas mixture to which the catalyst beads are exposed.
For purpose of their reduction dried impregnated beads may be directly placed into heater at 300xc2x0 C. with an ethylene-inert or 299xc2x0 C. with a hydrogen-inert gas flow mixture. Reducing gas flow has to be sufficient to provide complete reduction of the catalyst metals and may be varied in contact time range.
After about 10-15 minutes to about 5 hours of reduction in the gas mixture, the 1-5% ethylene or hydrogen in inert gas mixture may be turned off leaving the pure inert gas flow, and the beads may then be cooled back to room temperature. Shorter reduction times yield catalysts of higher STY values; hence shorter reduction times of from about 15 minutes to about 1 hour are preferred. Hydrogen is the preferred reducing gas.
The potassium promoters used in the process of this invention for producing the catalysts may include potassium alkanoates and any potassium compound that is converted to a potassium alkanoate during the alkenyl alkanoate-forming reaction (i.e., the reaction of ethylene, an alkanoic acid and an oxygen-containing gas in the presence of the catalyst to produce a alkenyl alkanoate). Potassium acetate is preferred as the promoter and is preferably added to the extent of about 5-10 wt. %, of the total catalyst weight on a dry basis. The promoter is preferably applied in the form of aqueous solutions. It is preferred to accomplish placement of the promoter on the reduced impregnated support beads by the xe2x80x9crotation immersion to a point of incipient wetnessxe2x80x9d technique as previously described.
The catalyst is dried to remove water.